Since I don't buy Hosta seeds I can't tell you of a source, but they
are relatively easy to grow from seed.
I simply scatter the seeds on top of a pot of quality potting mix,
water and wait. If the seed has been cold treated it will sprout
within a week to 10 days and be large enough to handle in about two
weeks. I then transplant the largest seedlings into Jiffy Peat Pellets
that have been previously soaked and expanded.
Hosta seedlings are painfully slow growing, but once they get started
they grow at a fairly even rate. By mid summer they should be large
enough to move on to quart sized nursery pots.
I plan to move the majority of the seedlings I have to 6-inch mum pots
right before I move them outside to harden them off and then they will
go into an area where they will be grown on to a salable size. They
will probably be moved on one more time before the end of summer. Nest
Spring they'll go into the next larger size pot.
PARKS SEEDS does sell a good many different types of perennial seeds so
you might try there for Hosta seed, or do a Google Search for Hosta